The human shoulder consists of three bones: the clavicle/collarbone, the scapula/shoulder blade, and the humerus/upper arm bone, and furthermore includes multiple muscles, ligaments and tendons forming the rotator cuff. The articulations between the bones of the shoulder make up the shoulder joints. “Shoulder joint” typically refers to the glenohumeral joint, which is the joint where the humeral head articulates in the glenoid fossa.
The area of articulation has white cartilage on the ends of the bones, the so-called articular cartilage, which facilitates low friction movement or articulation between the bones. The shoulder must be mobile enough for the wide range motions of the arms and hands, but also stable enough to allow for motion such as lifting, pushing and pulling. This combination of stability and mobility results in large muscle reaction forces to the joint articulation surfaces
Due to long term excessive loading, arthritis or trauma injury the cartilage may start to wear out which causes pain and stiffness. Due to rotator-cuff deficiency the joint may also lose its stability and therefore its functionality.
Shoulder replacement surgery is an option for treatment of unstable, non-functional and painful shoulder joints. The joint replacement surgery aims to relieve arthritic pain and to re-establish the functionality of the shoulder joint for daily activities. In a shoulder replacement surgical procedure the glenohumeral joint is partly or fully replaced by a prosthetic implant.
There are four common methods for shoulder replacement known in the art, namely: total shoulder replacement, reverse shoulder replacement, replacement with a hemi-prosthesis and replacement with a bi-polar prosthesis.
Total shoulder replacement involves the replacement of the ball and socket joint. An artificial head replaces the humeral head and a PE glenoid socket-like component replaces the cartilage on the glenoid cavity. Total shoulder replacement functions well in cases where the shoulder muscles, ligaments and tendons comprising the rotator-cuff show good functionality.
In cases where the rotator cuff is less functional a reverse prosthesis is used. The reverse prosthesis features a metal head or ball section that replaces the glenoid. The articulating socket is being implanted in the humerus. The method of reversing the socket and the head distalizes the humerus and medializes the centre of rotation, and herewith allows the deltoid muscle to compensate for the rotator cuff deficiency. Good functionality is reached with this method.
The hemi-prosthesis only replaces one half of the joint with an artificial surface. The humerus head is resected and replaced by a metal component. This metal component articulates against the natural glenoid.
More recently a bi-polar prosthesis has been introduced used in shoulder arthroplasty. A bi-polar prosthesis comprises a hemispherical humeral head, directly engaging with the glenoid, and a second ball-in-socket connection between the humeral head and the humeral shaft.
Both the bi-polar and a hemi-prosthesis are used to treat rotator cuff arthropathy of the shoulder in patients with low functional demands.
Different prosthesis for shoulder replacement are known in the art. For example, US 2006/0079963 A1 (Hansen Regan) discloses a shoulder replacement device for treatment of rotator cuff arthroplasty which comprises a glenoid component and a humeral component. The glenoid component is generally concave or cup-shaped and comprises structures for attaching the component to at least two, preferably three of the most lateral projections of the scapula, namely the acromion process, the coracoid process and the glenoid fossa. The glenoid component comprises two members which may be drilled into the bone. At a third location, the glenoid component may be anchored to the bone by means of bone cement. The humeral component comprises a generally spherical or hemispherical member which is anchored by a stem system into the humerus.
US 2009/062923 A1 (Swanson Todd) describes a method and apparatus for total shoulder arthroplasty. A glenoid component comprises a body having an outer surface configured to anchor to the scapula and to be located adjacent the clavicle and the acromion. The outer surface comprises at least one portion of tissue in-growing material, especially located in the area of contact to the scapula. In a first step, the glenoid component may be transiently anchored to the scapula by means of screws, pegs, bolts, wires or the like prior to the permanent anchoring accomplished by tissue and bone ingrowth.
EP 1 314 407 A1 (Sulzer Orthopedics Ltd.) discloses a shoulder prosthesis having a glenoid element with a cup-like bearing. The glenoid element is fixated to a bone of the shoulder by means of pegs which are secured in the bone with bone cement.
US 2009/0192621 A1 (Biomet Manufacturing Corp.) discloses an implant assembly for a shoulder joint which includes a planar base and a humeral stem. The implant assembly further comprises an adaptor to be arranged between said planar base and said humeral stem. The adaptor may comprise two different length axes such as to allow a displacement of the planar base and the humeral stem relative to each other.